1. Field
One embodiment of the present invention relates to a projection display device and a display method.
2. Description of the Related Art
Conventionally, projection display devices which use liquid crystal light valves and digital micromirror devices (DMDs) are known. For example, a projection display device uses a projective liquid crystal panel (liquid crystal light valve) as light modulating means. Such a display device separates white light emitted from a light source into three primary color light beams, modulates the respective separated color light beams via the liquid crystal light valve so as to project an optical image transmitted through the liquid crystal light valve onto a screen via a projection lens.
In such a projection display device, when an optical image is projected from a front of a screen with a projection device aligned with a center of the screen, a rectangular-shaped optical image is projected because an optical axis is perpendicular to the screen. In most of devices which are called as front projectors for projecting from a front of the screen, however, the optical axis of the projection lens and the center of the light valve are shifted to up-down and right-left directions by any amount in order to allow all observers to easily see a projected image. In such a manner, offset projection is carried out optically in any direction. It is, however, difficult to realize a large shift using optical means.
In order to further shift a projection range, therefore, a projection device is arranged below the center of the screen, and the optical axis of the projection lens is directed obliquely upward so that extended projection is carried out. This is one kind of projection systems which is generally called as “shifting projection”, and in this system, an image is projected above the center of the projection lens. The shifting projection includes vertical shifting projection for projecting an image upward, horizontal shifting projection for projecting an image in a horizontal direction, and projection where they are mixed.
In the case of the upward shifting projection, an image projected onto a screen extend upward toward the upper portion, and are widened in a horizontal direction so that the image is distorted into an inverted trapezoidal shape (keystone distortion). It is considered that the keystone distortion due to the shifting projection is corrected by a signal processing (for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-120614 (paragraphs 0005 to 0007), and Jpn. Pat. Appln. KO AI Publication No. 2007-17537 (paragraphs 0003 to 0004)).
In the keystone correction described in these two patent documents, an optical image which transmits through a liquid crystal light valve is distorted in advance in a manner opposite to a distortion, and thus a rectangular image is projected. That is to say, since a rectangular image is distorted into an inverted trapezoidal image when it is projected, the optical image is corrected to have a trapezoidal shape before it is projected. The optical image is, however, corrected so that a lower side of an inverted-trapezoidal projectable area (short boundary side) matches a lower side of a corrected rectangular projected image. For this reason, the degree of a reduction in the optical image due to the correction does not become a problem within a general projection distance where a focal distance is long. However, in short-focus display devices which are applied to intelligent boards and whose maximum field angle exceeds 40°, the entire device should be greatly tilted due to a shift in a slight projection range (tilt angle: about 25.1°), and also a projected image after keystone correction becomes a small part of a projectable area where the display device can display an image. An effective light flux is reduced (image darkens), and the number of effective display pixels is reduced (image definition deteriorates). Further, various secondary issues are likely to arise due to presence of a large unnecessary light (stray light) area.